Beds as opposed to seats mean fewer passengers per train carriage, so the fixed cost of operating the train is shared among fewer tickets.
You can make one trip per day with a night-train, whereas a seated train runs 16-20hrs per day. So again, fixed cost of the train existing is shared between far fewer ticket sales.
Then you need to store the train during the day, and you need to securely park the train when it arrives at its destination at night (a normal train yard isn't secure enough if you have a hotel full of people, and a train station usually needs all of its platforms).
You can make one trip per day with a night-train, whereas a seated train runs 16-20hrs per day. So again, fixed cost of the train existing is shared between far fewer ticket sales.
Then you need to store the train during the day, and you need to securely park the train when it arrives at its destination at night (a normal train yard isn't secure enough if you have a hotel full of people, and a train station usually needs all of its platforms).